Depends on the mod and what it does. Mods in Silent Hunter aren't usually different "game modes" like they are in other games, but instead change the UI or functions of the game to play differently.
Like you might have a mod that makes your torpedoes do more damage, or makes prettier sunsets, or one that adds more commands to your UI, or more realistic scopes, or one that makes the enemy destroyers far more dangerous, or one that simply makes the soup in your galley steam.
Some mods are really obvious that they are active, while others are more subtle or work in the background so you can't easily see what they are doing.
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